Theoretical Investigation of Selenium Interferences in Inductively Coupled Plasma Mass Spectrometry G. Bouchoux,* ,† A. M. Rashad, ‡,§ and A. I. Helal ‡ † Laboratoire des Mé canismes Ré actionnels, Dé partement de Chimie, Ecole Polytechnique and CNRS, 91128 Palaiseau, France ‡ Central Laboratory for Elemental and Isotopic Analysis, Nuclear Research Center, Atomic Energy Authority, Cairo 13759, Egypt * S Supporting Information ABSTRACT: Structures, heats of formation, ionization energies, and proton affinities of selenium, argon dimer, argon-chlorine, and their hydrides (Se, SeH, SeH 2 , ArH, ArH 2 , Ar 2 , Ar 2 H, Ar 2 H 2 , ArCl, and ArHCl) are estimated by quantum chemistry calculations using G3, G4, and W1 composite methods and coupled cluster approach at the CCSD(T)/aug-cc-pVTZ levels. Thermochemistry of the reactions between ions A + = Se •+ , SeH + , SeH 2 •+ , SeH 3 + , Ar 2 •+ , Ar 2 H + , Ar 2 H 2 •+ , Ar 2 H 3 + , ArCl + , ArClH •+ , and ArClH 2 + with various neutral gas G commonly used in dynamic reaction chamber-inductively coupled plasma-mass spectrometry (DRC-ICP-MS) (G = H 2 , CH 4 , NH 3 ,O 2 , CO, CO 2 , NO, and N 2 O) has been investigated. ■ INTRODUCTION Selenium is an element found in mineral form in sulfide ores (mainly pyrite) and in many organic species, particularly in foods (Brazil nuts, mushrooms, eggs, shellfishes, liver, kidney, etc.). 1-3 Selenium is mainly used, at the industrial level, in glass manufacturing and as catalyst, pigment, or alloys component. 2 Because of its fungicidal properties, selenium is also used in dermatology (antidandruff shampoo and body lotions). In human, selenium is present in selenoproteins (containing selenocysteine or selenomethionine), which act as antioxidant enzymes (glutathione peroxidase and thyroid hormone deiodinase). If at trace level, selenium is an essential element for animals and plants; it is, however, toxic at large doses (the present Tolerable Upper Intake Level is fixed to 400 μg per day). 1,4 Determination of selenium by sensitive and accurate analytical methods in water, soils, and biological fluids is consequently of importance. The usual methods are atomic absorption 5 and fluorescence detection 6 and inductively coupled plasma/mass spectrometry (ICP-MS). 7 Limitations in ICP-MS detection and quantization of selenium are due to interfering isobaric polyatomic cations. 7-9 Accordingly, the six isotopes of selenium 74 Se, 76 Se, 77 Se, 78 Se, 80 Se, and 82 Se (relative natural abundances: 0.89, 9.37, 7.63, 23.77, 49.61, and 8.73%, respectively) are isobaric with argon containing ions such as Ar 2 •+ , Ar 2 H + , Ar 2 H 2 •+ , ArCl + , and ArClH •+ and, obviously, with selenium hydrogenated derivatives SeH + , SeH 2 •+ , and SeH 3 + (Table 1). Two types of experimental setups are generally used to separate spectral interferences: (i) sector field mass spectrom- eters operated in high-resolution mode and (ii) quadrupole or time-of-flight mass analyzers coupled with collision/reaction cells. 7-9 However, under high resolution setting, the former instruments are generally less stable and less sensitive, thus leading to variable detection limit. 8 For this reason, collision cell technologies have been developed and increasingly used in analytical plasma mass spectrometry in the last decade. These techniques in which a gas-filled multipole is located before the mass analyzer allow the elimination of interference by ion- molecule reactions or by collision induced dissociations. Strictly speaking, a collision cell is a device where part of the kinetic energy of the incoming ion is converted into internal energy after inelastic collision with the target gas. It is generally operating at low pressure and high kinetic energy of the incoming ions in order to induce its activation by transfer of a part of kinetic energy to internal degrees of freedom. Under these conditions, collision cells allow endothermic fragmenta- tions to occur. By contrast, in a reaction cell, high pressure and low kinetic energies are operated in order to promote collision energy damping in order to attain near thermal conditions. Ideally, reaction cells allow exclusively the occurrence of exothermic chemical (ion-molecule) reactions. Ion-molecule reactions that may occur with the target gas in the gas cell of an ICP-MS device involve either the interfering ions or the element cation to be analyzed. Target gases such as NH 3 , 8,10 CH 4 , 11-13 H 2 , 14,15 and mixtures of H 2 in mixture with He 7,8 have been used to remove the Ar 2 + interfering ions. Shift of the Se + signal to SeO + by oxidizing agents such as O 2 or N 2 O has been also reported. 8,9,15 The mechanisms that may be involved inside the reaction/collision cell are charge exchange or proton, hydride ion, or atom (H, O, etc.) transfers, depending upon the element cation and the reaction gas. Understanding and predicting these reactions would be obviously facilitated by Received: May 14, 2012 Revised: August 21, 2012 Published: August 23, 2012 Article pubs.acs.org/JPCA © 2012 American Chemical Society 9058 dx.doi.org/10.1021/jp304644u | J. Phys. Chem. A 2012, 116, 9058-9070